1. Field of the Invention
The present invention relates to EMI filters. More particularly, the present invention relates to an EMI filter for use in an electric vehicle propulsion system. While the invention is subject to a wide range of applications, it is especially suited for use in electric vehicles that utilize batteries or a combination of batteries and other sources, e.g., a heat engine coupled to an alternator, as a source of power, and will be particularly described in that connection.
2. Description of the Related Art
For an electric vehicle to be commercially viable, its cost and performance should be competitive with that of its gasoline-powered counterparts. Typically, the vehicle's propulsion system and battery are the main factors which contribute to the vehicle's cost and performance competitiveness.
Generally, to achieve commercial acceptance, an electric vehicle propulsion system should provide the following features: (1) vehicle performance equivalent to typical gasoline-powered propulsion systems; (2) smooth control of vehicle propulsion; (3) regenerative braking; (4) high efficiency; (5) low cost; (6) self-cooling; (7) electromagnetic interference (EMI) containment; (8) fault detection and self-protection; (9) self-test and diagnostics capability; (10) control and status interfaces with external systems; (11) safe operation and maintenance; (12) flexible battery charging capability; and (13) auxiliary 12 volt power from the main battery. In prior practice, however, electric vehicle propulsion system design consisted largely of matching a motor and controller with a set of vehicle performance goals, such that performance was often sacrificed to permit a practical motor and controller design. Further, little attention was given to the foregoing features that enhance commercial acceptance.
For example, a typical conventional electric vehicle propulsion system comprises, among other things, a power bridge including high-power switching transistors for supplying current to the windings of a motor. In operation, the power bridge rapidly switches high currents from the power source creating substantial EMI as, for example, voltage spikes, harmonic currents, and parasitic oscillations. This conductive EMI will cause power conductors interconnecting the power bridge and other components to act as radiators, emitting radiative EMI that can interfere with on board electronic equipment such as computers and radio receivers. Likewise, in a high voltage system such as an electric propulsion system for an electric vehicle, conductive EMI can also disrupt systems operations and may damage or degrade system components.
Moreover, conventional electronic filter elements, such as an active snubber capacitor assembly, generally include resistive elements to compensate for stray inductance in the filter circuit. Such resistive elements denigrate system efficiency and generate additional heat energy. Thus, in an electric vehicle propulsion system where high efficiency and self cooling are highly desirable, such resistive elements are disadvantageous.